This project is concerned with mechanisms underlying development of the nervous system. From a developmental point of view, the two most striking features of the nervous system are the heterogeneity of cell types and the specificity of connections between the cells. We plan to investigate aspects of both of these features in a rapidly developing nervous system which can be cultured, has many identified cells, is amenable to morphological, physiological, and biochemical analysis, and is relatively simple. Two specific problems will be analyzed in these experiments: first, we have described the origin and morphological differentiation of a pair of "peripheral pioneeer neurons" which form the original pathway between the sensory nervous system and the central nervous system. The axons of these cells grow along a sterotyped path across a morphologically homogeneous sheet of cells in an accessible location. We intend to examine axonal guidance with intracellularly injected fluorescent markers, cell translocations and transplants, and surgical rearrangements of the epithelium. Secondly, an abundance of neuron types are generated in this system by a two-dimensional array of individually-identifiable neuron precursors. We plan to analyze the roles of position and lineage in determining cell type using intracellular markers, deletions, and translocations.